In Solem v. Bartlett, 465 U.S. 463 (1984), the Supreme Court, in determining whether South Dakota had criminal prosecutory authority over a member of the Cheyenne River Sioux Tribe, considered whether a reservation has been “diminished” by a subsequent act of Congress. The Supreme Court developed the diminishment doctrine in response to the General Allotment (or Dawes) Act of 1887, and the difficulties attendant to interpreting post-treaty “surplus lands acts” enacted by Congress to open certain reservations to settlement.

The Dawes Act envisioned the division of reservations into smaller family-size farms, or allotments. The remaining lands within a reservation were to be declared “surplus” or “opened” and made available to non-Indians. The Dawes Act included no specific time frame or instructions for how its general intentions should be implemented. Accordingly, Congress followed the Dawes Act with a series of surplus lands acts that allotted and opened particular reservations. Over 100 surplus lands acts were enacted.

The methods used by the surplus lands acts to open reservations to settlement by non-Indians varied. In some instances, Congress codified agreements reached between the United States and Indian tribes. In other cases, the acts were drafted regardless of dissent by tribes and their members. In either case, surplus land acts generally did not specifically address the fundamental question often raised during attempts to implement such acts — whether Congress, in opening surplus lands of a reservation, sought to remove those lands from the reservation and thus accomplish a diminishment or disestablishment of the reservation’s boundaries.

In order to distinguish between those post-treaty statutes that removed lands from the reservation from those that merely made “surplus” lands available for settlement within the reservation, the courts developed the diminishment doctrine. The Supreme Court in Solem summarized the basic legal principles that determine whether an Indian reservation has been diminished:

(1) “Once a block of land is set aside for an Indian reservation, and no matter what happens to the title of individual plots within the area, the entire block retains its reservation status until Congress explicitly indicates otherwise.”

(2) “Diminishment . . . will not be lightly inferred. Our analysis of surplus lands acts requires that Congress clearly evince” an intent to change boundaries before diminishment will be found.

(3) “The most probative evidence of Congressional intent is the statutory language used to open the Indian lands. Explicit reference to cession or other language evidencing the present and total surrender of all tribal interests strongly suggests that Congress meant to divest from the reservation all unallotted opened lands.”

(4) “Explicit language of cession and unconditional compensation are not prerequisites for a finding of diminishment. When events surrounding the passage of a surplus land act . . . unequivocally reveal a widely-held, contemporaneous understanding that the affected reservation would shrink as a result of the proposed legislation, we have been willing to infer that Congress shared the understanding that its action would diminish the reservation . . . .”

(5) “To a lesser extent, we have also looked at events that occurred after the passage of a surplus land act to decipher Congress’ intentions. Congress’ own treatment of the affected areas, particularly in the years immediately following the opening, has some evidentiary value, as does the manner in which the Bureau of Indian Affairs and local judicial authorities dealt with unallotted, open lands.”

(6) “On a more pragmatic level, we have recognized that who actually moved into an opened reservation is also relevant to deciding whether a surplus land act diminished a reservation. Where non-Indian settlers flooded into the opened portion of a reservation and the area has long since lost its Indian character, we have acknowledged that de facto, if not de jure, diminishment may have occurred.”

(7) “When both an act and its legislative history fail to provide substantial and compelling evidence of a Congressional intention to diminish Indian lands, we are bound by our traditional solicitude for the Indian tribes to rule that diminishment did not take place and that the old reservation boundaries survived the opening.”

Applying these legal principles, the Court concluded that “[n]either the Act [that opened the reservation], the circumstances surrounding its passage, nor subsequent events clearly establish that the Act diminished the Cheyenne River Sioux Reservation. The presumption that Congress did not intend to diminish the Reservation therefore stands . . . .”